The invention concerns a method and a device for producing slabs in a continuous casting installation, with an oscillating casting mold and a downstream strand guide below it, in which the cast strand is bent from the vertical casting direction into the horizontal rolling direction and during this process is supported and conveyed by driver rolls, which are arranged opposite each other in pairs, are adjusted relative to each other with well-defined contact force and can be combined into segments, and is deformed by at least one pair of driver rolls to a thickness that is reduced relative to its cast state, after which the continuous preliminary section or the reduced strand is cut into slabs, which are conveyed to a soaking furnace and then to a rolling mill.
So that the cast strand, which is produced in a continuous casting installation with a thickness of less than 100 mm, can be conveyed out of the continuous casting installation, the driver rolls are pressed against the strand with a certain pressure which prevents the driver rolls from slipping through and produces a sufficiently large tensile force on the strand below the point of complete solidification. In the state of the art, this pressure of the driver rolls in the area of complete solidification or locally sooner is utilized to alter the strand thickness, since the rolling forces to be applied are small due to the fact that the cast strand is still soft.
For example, DE 38 22 939 C1 describes a continuous casting method for the production of slabs with a reduced thickness relative to the cast state, in which a strand whose cross section is partially solidified is deformed by rolls that can be hydraulically adjusted relative to each other. These rolls acts to deform the strand both within the solidification section and in the area of the completely solidified strand, and during this process, the strand is deformed from about 60 mm to a final gage of 20 to 15 mm, and at the same time a product with a high proportion of rolling microstructure is produced. In this regard, at least one pair of rolls that acts on the already completely solidified part of the strand can be adjusted against stops to ensure the final dimension of the strand.
DE 198 17 034 A1 describes a method for the continuous casting of thin metal strip in a continuous casting installation with an oscillating, water-cooled mold, in which, directly after the complete solidification of the cast strand, at least one pair of driver rolls is continuously pressed against the strand with a variably defined pressure to achieve a well-defined thickness reduction of at least 2% and to maintain a desired strand thickness that has been adjusted in advance at a constant level.
Finally, EP 0 804 981 B1 describes a continuous casting method and a continuous casting device, in which cast slabs are fed to a large number of reducing installations, each of the reducing installations is assigned a target rolling reduction or a target pressure, and a deformation of a liquid core of the slabs is carried out, such that cast slabs can be produced with increased or decreased thickness compared to the slabs continuously removed from the mold.
In addition to the effort to reduce the thickness of the cast strand inexpensively and with relatively simple means that are already available by using the drivers that are already present, another objective that needs to be pursued is improvement of the surface quality of the slabs that are produced. In their cast state, continuously cast products may have surface defects, such as oscillation marks and other microstructural inhomogeneities. Subsequent rolling of the slab into a strip then results in defects in the strip surface. The effect of oscillation marks in austenitic steels consists essentially in the fact that, at the base of the oscillation marks (in the notch), there is diminished heat dissipation, which results in coarsening of the microstructure and segregation. These are mainly Cr or Mo concentrations. These concentrations lead to the formation of intermetallic phases, which, as the cause of the specified surface defects, must be removed by grinding before the rolling operation is carried out.
The solidification behavior of austenites is characterized by shrinkage during the transformation from ferrite to austenite, which results in a tendency of the strand shell to contract. This contraction can lead to increased delta ferrite concentrations and to poorer hot workability in the affected places. The nonuniform solidification at the surface then causes so-called scale patterns during direct rolling. These negative phenomena also generally have to be eliminated by grinding.
In ferritic steels as well, oscillation marks cause diminished heat dissipation at their base, which results in coarsening of the microstructure and segregation (Ni concentration, hard spots). To obtain a satisfactory final product, these inhomogeneities must also be eliminated by grinding.
The aforementioned surface defects cannot be eliminated by the previously known deformation of the cast strand while it is still soft, since the practical effect is to “knead” especially the oscillation marks that are present more deeply into the soft cast strand.